The invention relates to inspection of printed circuit boards, and it relates, more particularly, to the type of inspection which reveals whether the proper components have been properly and correctly installed on the board.
In mass-production of printed circuit boards, the boards are first loaded with components and then wave soldered. It is more expensive to repair a board after soldering than before soldering. Consequently, it is hightly advantageous to examine the board before soldering to verify that the proper components are installed in their designated locations with the correct polarities.
It is known to inspect printed circuit boards by measuring the brightness profile at selected locations on the board and comparing the result obtained with a standard derived from a properly loaded board. This technique has three problems, First, it is necessary to use precisely controlled illumination to make meaningful measurements. This is complicated and expensive. Second, the brightness profile of a component will vary as a result of sample-to- sample and manufacturer-to-manufacturer variations in surface finish. The brightness profile of a component may therefore be an unreliable indication of the identity of the component. Third, it can be difficult (as in the case of Dual-In-Line-Package, DIP integrated circuits) to inspect the components for proper physical orientation and electrical polarity using brightness information alone.
One subject of the invention is to provide a device for inspecting printed circuit boards which does not require controlled illumination.
Another object is to provide such a device which is unresponsive to sample-to-sample and manufacturer-to-manufacturer variations in surface finish.
A further object is to provide an arrangement capable of ascertaining the polarity or proper electrical connection of selective components by detecting their physical orientation.
Still a further object is to provide such a device which is inexpensive and robust by virtue of simplifying the photo detection requirements.